The present invention relates generally to the field of semiconductor inspection. More specifically, it relates to techniques for improving the sensitivity of defect detection on semiconductor samples, such as reticles and semiconductor devices.
Generally, the industry of semiconductor manufacturing involves highly complex techniques for fabricating integrating circuits using semiconductor materials which are layered and patterned onto a substrate, such as silicon. Due to the large scale of circuit integration and the decreasing size of semiconductor devices, the devices are ever increasingly sensitive to defects. That is, defects which cause faults in the device are becoming increasingly smaller. The device must be fault free prior to shipment of the device to the end users or customers.
An integrated circuit is typically fabricated from a plurality of reticles. Generation of reticles and subsequent optical inspection of such reticles have become standard steps in the production of semiconductors. Initially, circuit designers provide circuit pattern data, which describes a particular integrated circuit (IC) design, to a reticle production system, or reticle writer. The circuit pattern data is typically in the form of a representational layout of the physical layers of the fabricated IC device. The representational layout includes a representational layer for each physical layer of the IC device (e.g., gate oxide, polysilicon, metallization, etc.), wherein each representational layer is composed of a plurality of polygons that define a layer's patterning of the particular IC device.
The reticle writer uses the circuit pattern data to write (e.g., typically, an electron beam writer or laser scanner is used to expose a reticle pattern) a plurality of reticles that will later be used to fabricate the particular IC design. A reticle inspection system may then inspect the reticle for defects that may have occurred during the production of the reticles.
A reticle or photomask is an optical element containing at least transparent and opaque regions, and sometimes semi-transparent and phase shifting regions, which together define the pattern of coplanar features in an electronic device such as an integrated circuit. Reticles are used during photolithography to define specified regions of a semiconductor wafer for etching, ion implantation, or other fabrication process. For many modern integrated circuit designs, an optical reticle's features are between about 1 and about 5 times larger than the corresponding features on the wafer (e.g., fir laser lithography systems). For other exposure systems (e.g., x-ray, e-beam, and extreme ultraviolet), a similar range of reduction ratios also apply.
After fabrication of each reticle or group of reticles, each reticle is typically inspected by illuminating it with light emanating from a controlled illuminator. A test image of a portion of the reticle is constructed based on the portion of the light reflected, transmitted, or otherwise directed to a light sensor. Such inspection techniques and apparatus are well known in the art and are embodied in various commercial products such as many of those available from KLA-Tencor Corporation of San Jose, Calif.
During a conventional inspection process, the test image of the reticle is typically compared to a baseline image. Typically, the baseline image is either generated from the circuit pattern data or from an adjacent die on the reticle itself. Either way, the test image features are analyzed and compared with corresponding features of the baseline image. That is, an edge position within the test image is subtracted from a corresponding edge position within the baseline image to calculate a difference value. Each difference value is then compared with a predetermined threshold value. If the test image feature varies from the baseline feature by more than the predetermined threshold, a defect is defined and an error is reported.
Since defects on a reticle may be reproduced or printed on the resulting integrated circuit pattern, it is important to detect defects in each reticle prior to using the reticle to manufacturing integrated circuits. If defects are found in the reticle, the reticle can then be repaired or discarded before the reticle can produce faulty semiconductor devices. Unfortunately, all discrepancies on a reticle do not have a same effect on the corresponding integrated circuit pattern. For instance, not all reticle discrepancies are printed to the same degree on the wafer the same during the photolithography process. Additionally, some reticle discrepancies cause faults in the semiconductor device, while others do not. Whether the reticle discrepancy results in a printable error or fault on the integrated circuit depends on defect characteristics, such as size of the reticle discrepancy and characteristics of patterns which are proximate to the defect, such as density.
As described above, typical inspection processes detect defects on the reticle by comparing two areas on one or more reticles which are designed to be identical. When the difference is higher than a predetermined threshold, the difference is classified as a defect. Each difference detected between the two areas has the potential of resulting in a printable defect. Conversely, some of the detected defects will have no effect on the resulting integrated circuit. Although the threshold is typically set to capture defects having a size that will likely result in an actual integrated circuit defect, this technique sometimes fails to capture small defects that fall within densely packed active regions and, accordingly, tend to result in faults. Additionally, using a fixed threshold value does not account for different reticle patterns having varying degree of susceptibility to defects. That is, inspection of the reticle does not consider that one reticle pattern area will likely be significantly more susceptible to defects than a different reticle pattern area.
Accordingly, there is a need for improved inspection techniques for more accurately and reliably detecting defects in reticles, as well as detecting defects in the resulting integrated circuit. Additionally, it would be desirable to have techniques for accounting for the differences in defect susceptibility inherent in different reticle patterns